The ubiquitous interleukin-6: a time for reappraisal.

Interleukin-6 (IL-6) is a multifunctional cytokine regulating humoral and cellular responses and playing a central role in inflammation and tissue injury. Its effects are mediated through interaction with its receptor complex, IL-6Rβ (also known as gp130). It plays an important role in the pathogenesis of coronary artery disease and large quantities of IL-6 are found in human atherosclerotic plaques. IL-6 levels positively correlate with higher all-cause mortality, unstable angina, left ventricular dysfunction, propensity to diabetes and its complications, hypertension, obesity and several types of cancer. IL-6 levels augmentation demonstrates a remarkable parallel with another biomarkers reflecting harmful processes, like tumor necrosis factor alpha, interleukins 8 and 18, YKL-40, C reactive protein and resistin. Due to these facts, IL-6 was classified as a noxious interleukin. Nonetheless, there are several facts that challenge this usually accepted point of view. Since IL-6 has also anti-inflammatory activity, it seems reasonable to assume that favorable aspects exist. These aspects are two: 1. protection against bacterial infections, inactivating proinflammatory mediators, mitigating the course of septic shock and inducing the production of cortisol; and 2. influence on insulin sensitivity during exercise; this aspect is even more important. During exercise IL-6 is synthesized and released by muscles, with enhanced insulin action immediately at early recovery. Skeletal muscle may be considered as an endocrine organ; contracting muscles produce IL-6 and release it into the blood exerting its effects on other organs. The increase in circulating levels of IL-6 after exercise is consistent and proportional to exercise duration, intensity, muscle mass involved and endurance capacity. Thus, the fascinating possibility that the plenteous beneficial health effects of exercise could be ultimately mediated by IL-6 merits further elucidation. Interleukins were termed "good" or "bad", probably due to a tendency to see things in black and white, with no gray area in between. Calling IL-6 "a molecule with both beneficial and destructive potentials" would be a more equitable approach. In the literary creatures of Dr. Jekyll and Mr. Hyde, a good and an evil personality are found in the same individual. IL-6 playing the role of Dr. Jekyll is emerging; the time for IL-6 reappraisal is coming.